1. Field of the Invention
The present invention relates to a passive autofocus system for cameras which use both a passive phase-difference detecting method and a passive contrast detecting method.
2. Description of the Related Art
As conventional focus detecting methods used by autofocus systems for electronic cameras, the active autofocus method using triangulation and the passive autofocus method are known in the art. In addition, a phase-difference detecting method and a contrast detecting method are each known as the passive autofocus method in the art.
The active autofocus method using triangulation uses, e.g., an infrared emitter and a position sensitive detector (PSD) so that an object distance is determined by an application of triangulation, wherein the position sensitive detector receives an infrared signal which is firstly emitted by the infrared emitter and subsequently reflected by an object to be finally incident on the position sensitive detector to detect a position of the incident infrared signal. According to this active autofocus method using triangulation, the time necessary for determining an object distance is short, and the focusing lens group can be driven in a single stroke because an object distance is determined in a straightforward manner, which only requires a short time for an autofocus process. However, according to the active autofocus method using triangulation, an object distance cannot be measured with a high degree of accuracy. Moreover, the area on an object on which infrared is projected by the infrared emitter is limited to a small area because a focus detection area is small and because the infrared-projecting direction is fixed, so that there is a tendency for the foreground to be out-of-focus (i.e., the subject is out-of-focus) while the background is in focus.
In the phase-difference detecting method, a light bundle of an object image, which is passed through a photographing optical system to be formed on a reference focal plane in a predetermined focus detection area thereon, is separated into two light bundles by an exit-pupil splitting method (phase-difference detecting method) to be formed as two object images on a line sensor (which includes an array of photoelectric converting elements) to detect a phase difference between the two object images on the line sensor, and an amount of defocus is determined through a predetermined defocus operation in accordance with the detected phase difference. The focusing lens group is moved to a position at which the amount of defocus becomes minimal. The phase-difference detecting method has the advantage of having a long distance measuring range. However, the focus detection area is small and fixed.
The contrast detecting method, which uses a captured video signal in an electronic camera, uses a characteristic of the video signal such that the amount of the high frequency components of the video signal reach a maximum in an in-focus state for the object at the same distance. Namely, according to the contrast detecting method, the focusing lens group is driven in small steps to detect high frequency components of the captured video signal, and an in-focus position of the focusing lens group is determined so that the amount of the high frequency components of the captured video signal reach a maximum. This makes it possible to achieve focus with a high degree of accuracy. However, since the image contrast is detected by moving the focusing lens group in small steps, it takes a long time to determine the peak contrast intensity (in-focus position); moreover, it takes more time to determine the peak contrast intensity if the focusing lens group is positioned far away from an in-focus position thereof.
In view of the above described defects in conventional focus detecting methods, a hybrid autofocus system which uses the active autofocus method using triangulation and the contrast detecting method has been proposed in Japanese Patent Publications H05-210042 and 2001-141984, and another hybrid autofocus system which uses the phase-difference detecting method and the contrast detecting method has been proposed in Japanese Patent Publication H07-43605.
The hybrid autofocus system disclosed in Japanese Patent Publications H05-210042 and 2001-141984 firstly measures an object distance using triangulation to determine an in-focus position (temporary in-focus position) of the focusing lens group, drives the focusing lens group to a predetermined position thereof with reference to the temporary in-focus position, and subsequently drives the focusing lens group stepwise with reference to the temporary in-focus position to determine the peak contrast intensity using the contrast detecting method.
The hybrid autofocus system disclosed in Japanese Patent Publication H07-43605 firstly determines an in-focus position (temporary in-focus position) of the focusing lens group using the phase-difference detecting method to move the focusing lens group to the temporary in-focus position, and subsequently drives the focusing lens group stepwise with reference to the temporary in-focus position to determine the peak contrast intensity using the contrast detecting method.
In either of these two conventional hybrid autofocus systems, a focusing operation using the contrast detecting method is performed over a fixed range of focal settings with reference to the determined temporary in-focus position regardless of the accuracy of the initial operation determining the temporary in-focus position. Specifically, it is known that the aforementioned temporary in-focus position determining operation is performed with a high degree of accuracy for a high-contrast object, and a low degree of accuracy for a low-contrast object, according to each of the phase-difference detecting method and the contrast detecting method, both of which use an image contrast. Due to this characteristic, conventional focus detecting systems, which carry out a focusing operation using the contrast detecting method, perform a search through a superfluously wide range of focal settings for a high-contrast object. Moreover, such conventional focus detecting systems perform a search through an extremely narrow range of focal settings for a low-contrast object, which may cause the peak contrast intensity to be slightly or far out of the searching range to thereby make it impossible to determine the peak contrast intensity.